Adjustable resistance unit



Feb. 14, 1961 1.. M. BLOM, JR 2,972,123

ADJUSTABLE RESISTANCE UNIT Filed June 10, 1959 2 Sheets-Sheet 1 IN V EN TOR.

Feb. 14, 1961 BLQM, JR 2,972,123

ADJUSTABLE RESISTANCE UNIT 2 Shets-Sheet 2 I Lil g w Filed June 10, 1959 IN V EN TOR 2,972,123 ADJUSTABLE RESISTANCE UNIT Leonardus M. Blom, Jr., Covina, Calif., assignor to Con- Elco, Monrovia, Calif., a corporation of California Filed June 10, 1959, Ser. No. 819,324

'4 Claims. (Cl. 338-181) This invention relates to adjustable resistance units, and has particular reference to wire wound units so con structed as to be adjustable with infinite resolution rather than adjustable in steps only.

The conventional wire wound resistance unit includes a wire winding having a longitudinal axis about which successive turns are arranged in exposed side by side relationship, a cooperating wiper for making adjustable electrical contact with the exposed turns of the winding and suitable means for adjustably positioning the wiper along a path parallel to the axis of the winding so as to move the wiper across the turns of the winding. This simple motion of the wiper in the wire wound system incurs the disadvantage that the resistance is adjustable in steps only as the wiper moves across the turns of the winding. Nevertheless, the wire wound system affords the best available combination of other desirable characteristics such as small size, high available resistance, high allowable power dissipation, high allowable operating Leinperature, mechanical ruggedness, and operating sta- 1 ity.

It is possible to make a wire wound adjustable resistance which is adjustable with infinite resolution over the range of its winding, such as by mounting the wiper for spiral movement within the winding so that it follows the direction of the wire without crossing over the turns. Such conventional arrangements are however relatively complicated mechanically, and are of limited application because of their physical size and resistance range limitations.

In accordance with the present invention, there is provided a wire wound electrical resistance unit which incorporates a simple wiper motion and a wide resistance range over which there is infinite resolution of adjustment, these characteristics being incorporated in a fashion which perimts a miniature sealed constructon. This is accomplished by the employment of a resistor series which includes two variable resistors connected in electrical series and supported within a closed housing. The first variable resistor includes a resistance element in the form of a wire winding having a longitudinal axis about which succesive turns are arranged in exposed side by side relationship, a cooperating wiper, and means operable from the exterior of the housing for adjustably positioning the wiper along a path parallel to the axis of the winding so as to move the Wiper across the turns of the winding. The second variable resistor includes a resistance element in the form of a conductive strip supported within the housing alongside of and parallel to the axis of the winding, a cooperating wiper, and means operable from the exterior of the housing for adjustably positioning the wiper along a path parallel to the strip so as to provide infinite resolution between the wiper and the strip. The strip has a length comparable to that of the winding, and has a total electrical resistance which is relatively small compared to that of the winding and which is about the same as the resistance of one turn of the winding. Means are provided for making electrical connection to the serially connected first and second variable resistors from the exterior of the housing. i

In the preferred embodiment of the invention, a fixed resistor is included within the housing in series within the first and second variable resistors. The fixed resistor establishes a minimum resistance level for the unit, so as to keep the per turn resistance required of the winding down to a level which permits the use of a strip having a length comparable to that of the winding, a factor facilitating miniaturization.

A preferred embodiment of the invention is illustrated in the accompanying drawings in which:

Fig. 1 is a perspective view, partially cut away, of the preferred embodiment of the adjustable resistance unit of the invention;

Fig. 2 is a schematic drawing illustrating the electrical circuit of the resistance unit of Fig. 1;

Fig. 3 is a sectional elevation taken along line 3-3 of Fig. 1; t

Fig. 4 is a sectional elevation taken along line 44 of Fig. 1;

Fig. 5 is a fragmentary sectional elevation taken along line 5-5 of Fig. 1; and,

Fig. 6 is an end elevation of the housing taken along line 6-6 of Fig. 1.

Referring now to the drawing, a first variable resistor 10 and a second variable resistor 12 are mounted in parallel side by side relationship between opposite end walls 14, 16 of a closed housing 18.

The first variable resistor 10 includes a resistance element in the form of a wire winding 20, and a cooperating wiper 22. The wire winding 20 is wound spirally along the outer surface of the major portion of the length of a substantially hollow and cylindrical mandrel 24, so that the successive turns of the winding are arranged in exposed side by side relationship along the longitudinal axis of the mandrel. A slot opening 26 extends in a straight path along substantially the entire length of the mandrel, and a conductive backing member 28 is disposed over the slot on top of the winding 20, the turns of which also pass over the slot. The conductive backing member 28 is insulated from the winding 20.

Also, the mandrel 24 is an insulator.

The wiper 22 is mounted on a supporting sleeve 30 which is in turn supported on an adjustment screw 32. Extending upward from the sleeve 30 is a lug 34 which rides in the slot 26. The adjustment screw 32 extends axially within the mandrel, and is threaded throughout its length. The sleeve 30 has internal threads that mate with the external threads of the adjustment screw so that the contact 22 may be moved along a path parallel to the axis of the winding 20 by turning the adjustment screw 32. Rotation of the sleeve 30 is prevented by the lug 34 which rides in the longitudinal slot 26 in the mandrel.

The adjustment screw 32 is rotatably journaled at one end in a bearing member 36. At its opposite end, which is adjacent the end wall 14 of the housing, the adjustment screw has a slotted head 38 rigidly afiixed thereto. The head is rotatably journaled through the wall 14 of the housing in a mating opening 4!), and has a flange 42 at its inward end which rotatably fits a bore 44 of the mandrel.

A coil spring 46 is disposed coaxially about the head 38 and is caught between the flange 42 thereof and the wall 14 of the housing. Adjacent the wall 14 and surrounding the head member is a rubber O ring 48 backed by a washer 56. Thus, the spring 46 exerts pressure against the O ring 48 and also against the flange 42 of the head 38. This forces the distal end of the adjustment screw 32 against a conductive head 52 caught with? in the bearing member 36, andcauses the O ring to seal the opening 40 through which the head extends.

As can be seen in Figs. 1, 4 and 5, the basic construction of the second variable resistor 12 is essentially the same as that of the first variable resistor 10 described above With reference to Figs.'1' and 3. Thus, the second variable resistor includes an identical mandrel 54 having a longitudinal slot 56 through its upper wall, an identical lead screw 5% spring loaded and journaled at both ends in the same manner as in the first resistor, with an identical head member 60 journaled through the end Wall 14 of the housing. The adjustment screw 58 of the second variable resistor 12 carries an identical sleeve 62, lug 64 and wiper 66.

The resistance element of the second variable resistor is a strip or wire 68 of conductive material supported on the underside of a backing plate 70 which extends longitudinally of the mandrel 54 above the slot 56. The strip 68 is electrically insulated from the backing plate 70. It is cemented thereto.

Referring again to Fig. 3, further included in the resistor series within the housing is a fixed resistor 72 in the form of a wire winding wound around the mandrel 24- of the first variable resistor over a supporting sleeve 74 which separates it from the first resistor winding 20. The fixed resistor 72 maybe composed of as many turns wound in whatever manner desired, in order to provide a minimum resistance level for the unit.

First, second and third insulating wires 76, 78, 80 are sealed through the end wall 16 of the housing so as to provide electrical connection with the resistor series in the housing. The first insulated wire 76 is soldered to one end of the fixed resistor 72. The opposite end of the fixed resistor 72 is soldered to one end of the Winding of the first variable resistor 10. The opposite end of the winding 20 is soldered to the conductive backing plate 28. The second insulated wire 78 is also soldered to the conductive backing plate 28.

A fourth insulated wire 82 disposed entirely within the housing extends through the mandrel 24 and bushing member 36 to the conductive head 52. The wire 82 is soldered to the head 52 and runs therefrom to one end of the conductive strip 68 of the second variable resistor 12, to which it is connected.

The second variable resistor 12 includes a conductive bead 84 caught within a bearing member 86, similar to the structure of the first variable resistor. The third insulated wire 80 is soldered to the conductive bead 84 and runs therefrom through the end of the bearing member 86 and through the end wall 16 to the exterior of the housing.

Reviewing the connections it will be apparent that the fixed resistor 72 and the first and second variable resistors 10, 12 are connected in electrical series, such as is illustrated in Fig. 2. The primary electrical connection accessible from the exterior of the housing runs serially from the first wire 76 through the fixed resistor 72, the winding 20 of the first variable resistor 10 to the contact 22 of that resistor, through the sleeve which supports that contact and the lead screw 32 on which the sleeve rides to the conductive head 52, and from the conductive head 52 through the insulated wire 82 to one end of the conductive strip 68, thence through the contact 66 of the second variable resistor 12;, the sleeve 62 and the lead screw 58 of that resistor to the conductive bead 84, and on through the third wire 80 to the exterior of the housing.

The second wire 78 provides an additional terminal exterior of the housing by means of which the circuit may be employed as a potentiometer, or by means of which a rheostat circuit may be formed which by-passes the fixed resistor 72.

The parallel side by side relationship of the first and second variable resistors, and the coaxial disposition of the fixed resistor winding with the winding of the first variable resistor provides a rugged compact structure,

v 4 easily contained within a closed housing, with all external electrical connections being conveniently at one end of the housing and with the slotted heads of the lead screws being accessible at the opposite end of the housing for adjusting the position of the wipers along their respective resistance elements.

The first adjustable resistor 10 by virtue of its winding 20 provides a Wide resistance range, adjustable in steps. The second adjustable resistor 12, while having a small resistance range, is nevertheless effective to provide infinite resolution over the resistance range of the Winding 20, when employed in series with the first variable resistor 10. The resistance of the conductive strip 68 is at least as great as, and preferably equal to, the per turn resistance of the winding 20. The per turn resistance of the Winding is chosen so that the length of the conductive strip is comparable with that of the winding.

Typically, the winding and the conductive strip would each have a length of about 0.7 inch. The winding would have a total resistance of 44,000 ohms, with a per turn resistance of seventeen ohms. The conductive strip would have a total resistance of seventeen ohms.

The fixed resistor 72 included within the resistor series permits the total adjustment of the first and second variable resistors to be carried out beginning at a predetermined level. Typically, the fixed resistor would have a total resistance of 54,000 ohms, in a resistance unit designed to provide adjustment with infinite resolution between 54,000 ohms and 98,000 ohms.

I claim:

1. An electrical resistance unit comprising a closed housing, a resistor series which includes first and second variable resistors connected in series and disposed Within the housing, the first variable resistor including a resistance element in the form of a Wire winding having a longitudinal axis about which successive turns are arranged in exposed side by side relationship, a cooperating Wiper for making adjustable electrical contact with the exposed turns of the winding, and means operable from the exterior of the housing for adjustably positioning the wiper along a path parallel to the axis of the winding so as to move the wiper across the turns of the Winding, the second variable resistor including a resistance element in the form of a conductive strip supported within the housing alongside of and parallel to the axis of the winding, a wiper for making adjustable electrical contact with the strip, and means operable from the exterior of the housing for adjustably positioning the wiper along a path parallel to the strip so as to provide infinite resolution between the wiper and the strip, the strip having a length comparable to that of the winding and having a total electrical resistance approximately equal to the resistance of one turn of the Winding, and, means providing electrical connection with the resistor series from the exterior of the housing, whereby there is provided an electrical resistance unit the resistance of which is adjustable with infinite resolution over a wide range.

2. Apparatus of claim 1 further including within the housing a fixed resistor connected in series with the first and second variable resistors for establishing a predetermined minimum resistance level for the unit so as to keep the per turn resistance required of the winding down to a level Whichpermits the use of a strip having a length comparable to that of the winding.

3. Apparatus of claim 2 wherein the fixed resistor is in the form of a Wire winding disposed coaxially with one of the variable resistors.

4. An electrical resistance unit comprising a first resistance element in the form of a wire winding having successive turns supported in exposed side by side relationship, a cooperating first Wiper for making electrical contact with the winding, means mounting the first wiper for movement transversely across the turns of the winding, a second resistance element in the form of a strip of conductive material of about the same length as the winding and disposed approximately parallel to the axis of the winding, the strip having a total the resistance approximately equal to the resistance of one turn of the winding, means electrically connecting one end of said strip to the first wiper, a second wiper for making electri- 5 cal contact with the strip, and means mounting the sec- 0nd wiper for movement along the length of the strip so as to provide infinite resolution between the second wiper and the strip, whereby the electrical resistance between one end of the winding and the second wiper may be varied with infinite resolution over the range of the winding.

References Cited in the file of this patent UNITED STATES PATENTS 2,881,295 Brown Apr. 7, 1950 

